<!DOCTYPE html>
<html lang="zh-CN">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    html {
      font-size: 16px;
    }

    /* 满足特定条件时，对应的 CSS 生效 */
    @media (width: 375px) {
      html {
        font-size: 37.5px;
      }
    }

    div {
      width: 5rem;
      height: 5rem;
      background-color: red;
    }
  </style>
</head>

<body>
  <div></div>
</body>

</html>